This invention relates to liquid level gauges and more particularly to gasoline storage tank gauges capable of accurately and continuously monitoring and/or displaying the amount of gasoline and underlying water contained in a gasoline service station tank.
The capacity of such tanks is typically 8000 gallons. The tank length and height are 21 feet and 8 feet, respectively. At about half full, a change in level of 1 inch represents about 100 gallons.
The most accurate method now used for determining the level of a large volume of liquid is the wooden dipstick that is marked off in inches or other length units. It is inserted to the bottom of the tank and withdrawn. The position of the "wet line" is then noted. Due to evaporation and wicking, the resolution of this measurement is limited to about 1/4 inch.
A widely used gauge for measuring liquid level is comprised of two uniformly spaced electrodes that extend from the top to nearly the bottom of the tank. The dielectric constant of gasoline being greater than that of air, the electrical capacitance between the two electrodes is taken as a measure of the level. This method is not as accurate as the aforementioned dipstick method, being subject to errors due to the variability in gasoline vapor content of the air in the upper portion of the tank that changes the dielectric constant in this upper portion in an unpredictable way. It also raises problems of safety since it requires electrifying the electrodes that are exposed directly to the gasoline and the overlying highly flammable vapors.
Another liquid level gauge is comprised of a string of series connected magnetic field actuated reed-relays. A resistor is connected across each relay. The relay string and resistors are mounted in a protective tube which is mounted vertically in the tank. A magnet is mounted to an annular float that is adapted to be slidably movable about the tube in a vertical direction. The total resistance of the string is taken as a measure of the liquid level. This total resistance is inversely proportional to the number of reed relays whose contacts are closed. Like the above mentioned capacitor gauge, the reed relay gauge provides an analog measure of the liquid level.
It is particularly important for the operator of a gasoline service station that he be able to accurately detect changes in the level of his tanks. Losses through leakage, theft, or faulty accounting at filling are not likely to be detected by such gauges. These gauges have the disadvantage that they are especially insensitive to small level changes when the tank is from about half full to full of liquid.
It is an object of the present invention to overcome the above noted shortcomings of the prior art gauges.
It is a further object of this invention to provide a gasoline level gauge having the same resolution at the full condition as as the near empty condition.
It is a further object of the invention to provide such a gauge that is capable of continuous operation from a battery drawing a very small current so as to be capable of sounding an alarm in case of leakage or theft.
It is yet another object of this invention to provide such a gauge that may monitor the levels of two or more immiscible liquids of different densities in the same tank.